Modern approaches of producing X-ray images and X-ray tomography use more and more advanced methods for detection of the photons, which pass through the object or which is scattered in the object. However, they have typically no or very little knowledge of the photons, which are entered into the object. At best, only some average properties of the incoming photon beam are known.
Such asymmetry of the measurements, i.e. when only one end (the output) of two ends of the process (input and output) is well-measured, is generally disadvantageous and leads inevitably to inefficiency and to restrictions in the choice of detecting methods.
A simple example of such an inefficient approach is when an object having both dense parts almost opaque to X-rays and thin well-transparent parts is imaged. The image of well-transparent parts has low contrast and large noise even if the number of photons detected by each picture element is large. This can be understood mathematically: the number of absorbed photons, which reflects the density of the object, is the difference of two large numbers close to each other, i.e. the unknown (fluctuating) number of incoming photons and the number of outgoing photons as being transmitted through the object.